Republican presidential candidate Donald Trump at a campaign rally in Des Moines, Iowa, August 5, 2016. Fifty former national security officials wrote an open letter that called Mr. Trump unqualified to be president.

Fifty former national security officials, who served Republican administrations from Richard Nixon to George W. Bush, have signed an open letter saying Donald Trump would be a “dangerous president” because he “lacks the character, values and experience.”

“We know the personal qualities required of a president of the United States,” reads the letter, whose signatories include a former director of central intelligence and two former secretaries of homeland security. “We are convinced that in the Oval Office, he would be the most reckless president in American history.”

Candidates have been criticized for their lack of foreign policy experience before, including then junior Sen. Barack Obama of Illinois. The letter goes further, however. It blasts Mr. Trump’s temperament, impulsiveness, and morality.

“What we really wanted to do was to raise the alarm and awareness among voters who may find Mr. Trump attractive in a lot of ways, but do not understand what is required to be president of the United States,” John Bellinger III, writer of the letter’s first draft and a former legal adviser to Condoleezza Rice, told The Washington Post.

“We are trying to say to them, we have served inside the White House, we have worked with presidents for decades, we know what’s required to be president, and we are deeply concerned Donald Trump does not have these qualifications, the judgment or the temperament.”

In addition to Bellinger, signatories include Michael Hayden, a former director of central intelligence; Michael Chertoff and Tom Ridge, former secretaries of the Department of Homeland Security; and John Negroponte, former Director of National Intelligence. Absent are signatures from any of the living former secretaries of state from Republican administrations, including Ms. Rice, Colin Powell, and Henry Kissinger.

The letter emphasizes the importance of a president having a certain type of character and morality. It also describes the importance of a president’s abilities to listen, consider different views, and own mistakes.

“In our judgment, Mr. Trump has none of these critical qualities,” it reads. “He is unable or unwilling to separate truth from falsehood. He does not encourage conflicting views. He lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behavior.”

Trump, in response to the letter, said the signatories were partly responsible for Benghazi, the Iraq war, and the rise of the self-declared Islamic State.

“They are nothing more than the failed Washington elite looking to hold onto their power, and it’s time they are held accountable for their actions,” reads the statement. “Instead, I offer a better vision… that stands up to foreign dictators instead of taking money from them, seeks peace over war, rebuilds our military, and makes other countries pay their fair share for their protection.”

The lack of experience of other presidential candidates have been questioned before. President Obama and his 2008 Republican opponent, Sen. John McCain of Arizona, were criticized for how they might handle the Russo-Georgia war that year. Nor are such claims new for Trump.

“When he entered the race, the overarching concern was lack of experience and an inability or unwillingness to define what his policy would be,” Elliot Abrams, a Middle East expert and military hawk who served as deputy national security adviser to George W. Bush, told The Los Angeles Times.

As Trump laid out his foreign policy – in which he questioned the importance of NATO, spoke of his regard for Russian President Vladimir Putin, and immigration – some Republican foreign policy experts cringed, said Mr. Abrams.

In March, many of these experts signed an earlier version of an open letter against Trump. More than 100 members of the foreign policy establishment from the George H.W. Bush and George W. Bush administrations signed the letter that condemned Trump’s attitudes toward trade, immigration, Muslims, torture and his admiration of Mr. Putin of Russia. Many of this most recent letter’s signers told The New York Times they were reluctant to add their name to the spring letter.

That changed when Trump further questioned the importance of NATO and encouraged Russia to hack Hillary Clinton’s email, a remark that he later said he was a joke.

While those who signed the letter pledged to not vote for Trump, the letter didn’t endorse Ms. Clinton. The letter said they have their doubts about her, too.